1. Technical Field
The invention generally relates to the development of a common time reference value at all nodes on a local area network and more specifically relates to real time referencing on a ring topology local area network.
2. Background Art
The development of an accurate common time reference value at all the nodes on a local area network (LAN) is becoming increasingly important, particularly as the processing of a system becomes more distributed and takes on the characteristics of being "real time." There have been various schemes proposed for the distribution of a common time value to all the nodes on a LAN. Many of these operate as time reference software processes at the application layer (layer 7) of the International Standards Organization (ISO) Open Systems Interface (OSI) 7-layer communication model. FIG. 1 from "Computer Networks" by Tannenbaum, Prentice Hall 1988, defines the names and capabilities of the 7 layers of the ISO/OSI communications model. According to this model two or more layer 7 application processes communicate by utilizing the services provided by the lower layers of the model. Layer 1 represents the physical media which interconnects the nodes on a LAN. A message sent from one application process to another progresses down through layers 6,5,4,3 and 2 before being placed on the interconnecting media at layer 1. The message then traverses the physical media to the node(s) to which it was addressed, and then must traverse layers 2,3,4,5, and 6 before being presented to the target layer 7 application process(es). This communication path is graphically depicted in FIG. 2.
A software process operating at layer 7 of the ISO/OSI model does not have control nor visibility into the amount of software, firmware, and/or hardware processing time is expended in the transmission and reception of a message through the lower layers of the ISO/OSI communications model. As a consequence, a time reference value transmitted in a message from one node on a LAN will be offset considerably from the correct value of the current time when the message is received at a layer 7 time reference software process at other nodes on the LAN.
A report by David L. Mills entitled "Network Time Protocol Version 2 Specification and Implementation" is a sample of the current state-of-the-art in the area of network time distribution. However, methods such as those described by Mills have settling out times that can exceed 24 hours or more with one or two hours being typical. "A Probabilistic Approach to Distributed Clock Synchronization" by Falviu Cristian describes a second current state-of-the-art mechanism for the establishment of a system wide common time value. The Cristian process also operates at the application layer of the ISO/OSI model, and is not deterministic.